drm/amd/display: Add link_rate quirk for Apple 15" MBP 2017

This fixes a problem found on the MacBookPro 2017 Retina panel:

The panel reports 10 bpc color depth in its EDID, and the
firmware chooses link settings at boot which support enough
bandwidth for 10 bpc (324000 kbit/sec aka LINK_RATE_RBR2
aka 0xc), but the DP_MAX_LINK_RATE dpcd register only reports
2.7 Gbps (multiplier value 0xa) as possible, in direct
contradiction of what the firmware successfully set up.

This restricts the panel to 8 bpc, not providing the full
color depth of the panel on Linux <= 5.5. Additionally, commit
'4a8ca46bae8a ("drm/amd/display: Default max bpc to 16 for eDP")'
introduced into Linux 5.6-rc1 will unclamp panel depth to
its full 10 bpc, thereby requiring a eDP bandwidth for all
modes that exceeds the bandwidth available and causes all modes
to fail validation -> No modes for the laptop panel -> failure
to set any mode -> Panel goes dark.

This patch adds a quirk specific to the MBP 2017 15" Retina
panel to override reported max link rate to the correct maximum
of 0xc = LINK_RATE_RBR2 to fix the darkness and reduced display
precision.

Please apply for Linux 5.6+ to avoid regressing Apple MBP panel
support.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -3438,6 +3438,17 @@ static bool retrieve_link_cap(struct dc_link *link)
 		sink_id.ieee_device_id,
 		sizeof(sink_id.ieee_device_id));
 
+	/* Quirk Apple MBP 2017 15" Retina panel: Wrong DP_MAX_LINK_RATE */
+	{
+		uint8_t str_mbp_2017[] = { 101, 68, 21, 101, 98, 97 };
+
+		if ((link->dpcd_caps.sink_dev_id == 0x0010fa) &&
+		    !memcmp(link->dpcd_caps.sink_dev_id_str, str_mbp_2017,
+			    sizeof(str_mbp_2017))) {
+			link->reported_link_cap.link_rate = 0x0c;
+		}
+	}
+
 	core_link_read_dpcd(
 		link,
 		DP_SINK_HW_REVISION_START,